It’s hard to take Medicare for All seriously when its proponents keep saying deeply unserious things. The latest case in point comes from Rep.-elect Alexandria Ocasio-Cortez, the progressive firebrand from the Bronx.

In a Dec. 2 post on Twitter, she appeared to argue that Defense Department accounting irregularities could somehow pay most of the cost of a universal health plan.

“$21 TRILLION of Pentagon financial transactions ‘could not be traced, documented, or explained,’ ” she wrote, quoting a Nation magazine article. “Medicare for All costs ~$32T. That means 66% of Medicare for All could have been funded already by the Pentagon.”

That claim was spectacularly, obviously wrong, as multiple fact-checkers soon confirmed.

It turned out that $21 trillion was a tally of questionable accounting adjustments, both positive and negative, over 18 years — a measure of bad bookkeeping, but not necessarily waste or fraud. It was definitely not a pool of money available to be spent.

Indeed, some fact-checkers pointed out that $21 trillion is probably more, without adjusting for inflation, than the entire history of U.S. military spending.

This could be written off as a rookie mistake. But it fits a pattern of too-good-to-be-true arguments from the single-payer faithful.

Take Bernie Sanders himself, for example. Think tanks on the left and right agree that his Medicare for All plan would likely add about $32 trillion to federal spending over the first 10 years. Yet he keeps promoting the idea that it would cost less than half that amount, or $14 trillion.

In New York, likewise, a proposed state-run single-payer system is consistently portrayed as a free lunch: It would supposedly cover the uninsured, eliminate cost-sharing, allow an unfettered choice of doctors and hospitals — and still reduce overall health-care spending by billions.

The state Assembly has passed the bill four years in a row based on assurances from its author, Richard Gottfried of Manhattan, that it would cost $92 billion in new taxes. That translates to much more than doubling the state’s overall tax revenues, but even that estimate was way too low.

When the RAND Corp. took an independent look this summer, the estimated savings dwindled to almost nothing and the necessary tax hike mushroomed to $139 billion. Gottfried was off by a mere $47 billion.

Underlying these rosy scenarios are basic misconceptions about single-payer.

Supporters casually assert that single-payer is the norm in every other developed nation. In fact, countries including Switzerland, Germany and the Netherlands rely on a mixture of private and public insurance not unlike the structure of the Affordable Care Act — albeit with tighter mandates.

They insist that America’s highest-in-the-world health costs are primarily driven by the wasteful bureaucracy of insurance companies. According to federal data, however, the administrative expenses of private health plans account for less than 4% of national health expenditures. Plus, a portion of that spending goes toward holding down fraud and waste, and thereby pays for itself.

The overarching myth is that because single-payer countries spend less on health care than we do, switching to single payer would automatically control costs. This ignores the many other differences between their systems and ours, including generally lower salaries for health professionals and more restrained access to care.

In New York, for example, women are entitled to coverage for annual mammograms starting at age 40. Britain’s National Health Service, by contrast, normally offers breast-cancer screenings starting at 50, and then only once every three years.

This is not to suggest that single-payer is impossible. Britons are not dying in droves. But trying to graft their financing methodology onto our very different health-care culture and political system would be enormously complicated and expensive — and proponents should stop pretending otherwise.